1. Field of the Invention
Embodiments of the present invention generally relate to modulation of audio signals in a parametric speaker.
2. Description of the Related Art
Parametric speakers or arrays are known to be attractive sound reproduction systems because of their sharp directional characteristics. In general, a parametric speaker is a sound emission system that creates sound by emitting high frequency ultrasonic waves made up of a carrier frequency and frequencies from modulation of an audio signal with the carrier frequency. The ultrasonic frequencies are demodulated in air or another nonlinear medium to regenerate the audio signal. The parametric sound beam is highly directional and maintains audio intelligibility over long distances.
Parametric speaker technology is based on the well-known Berktay's ‘demodulation’ effect of ultrasonic propagation in the air which is described in H. O. Berktay, “Possible Exploitation of Nonlinear Acoustics in Underwater Transmitting Applications,” J. Sound & Vib. 2, pp. 435-461, 1965. More specifically, if the ultrasonic signalp1(t)=(1+s(t))sin(ωct)  (1)is emitted in the air, a far-end listener would perceive audio signal as
                                                        p              2                        ⁡                          (              t              )                                ∝                                                    ∂                2                                            ∂                                  t                  2                                                      ⁢                          (                              1                +                                  2                  ⁢                                                                          ⁢                                      s                    ⁡                                          (                      t                      )                                                                      +                                                      s                    ⁡                                          (                      t                      )                                                        2                                            )                                      ,                            (        2        )            where s(t) is the input audio signal, and ωc is the modulation carrier angular frequency high above the audio band. Eq. (2) suggests that the ‘demodulation’ effect reveals the input audio signal out of the ultrasonic signal, and also generates distortion due to the square term of the signal.
Various modified modulation techniques to use instead of Eq. (1) to eliminate the distortion term in Eq. (2) have been proposed. Two such techniques are described in T. Kamakura, et al., “Suitable Modulation of the Carrier Ultrasound for a Parametric Loudspeaker,” ACUSTICA, 73, pp. 215-217, 1991, and E. L. Tan, et al., “Distortion Analysis and Reduction for the Parametric Speaker,” Proceedings of the 124th AES Convention, May, 2008 (“Tan”). These techniques require that the transducer emit a wider ultrasonic bandwidth, which may be impractical in many cases. An alternative technique that cancels the distortion portion using simulated distortion is described in M. E. Spencer, et al., U.S. Pat. No. 7,729,498, entitled “Modulator Processing for a Parametric Speaker System,” filed Jan. 9, 2007 (“Spencer”). This technique requires a module that exactly simulates the ‘demodulation’ effect of Eq. (2) and the robustness of the technique is unknown.
The techniques described in Tan and Spencer and the technique described in S. Sakai, et al., “Dynamic SSB Modulator for Parametric Loudspeaker,” IEICE Trans. Fundamentals(A), vol. J91-A, No. 12, pp. 1166-1173, 2008 (“Sakai”), suggest use of a single sideband (SSB) frequency instead of the double sideband (DSB) frequencies of Eq. (1). Sakai explains the benefit of SSB, i.e., the square term in Eq. (2) is contributed by the upper and lower sidelobes generated by Eq. (2) and hence eliminating either may improve the sound quality. As is explained in Spencer and Y. Wang, et al., “Defining the Parameters of Truncated Square-rooting DSB for Parametric Loudspeaker,” Proc IEEE, pp. 1689-1693, August, 2007, a similar effect to that of SSB can be expected by using Truncated DSB (TDSB) frequencies.
The previously mentioned techniques are all based on amplitude modulation (AM). The Japanese patent application JP2006-135778 proposes systems that instead use pulse width modulation (PWM) that can be implemented with a simple switching hardware circuit. However, the PWM based techniques may produce inferior sound quality as they generate more harmonics than using AM.